2. Copying

This document may be redistributed, verbatim or in modified form, under
the terms of the GNU General Public License, version 2 or any later
version. The same terms apply to the libraries it documents. A copy
of the General Public License is provided as an Appendix.

Most XEmacs documentation has its own license, which is an ancestor of
the GNU Free Documentation License (FDL), and whose terms are
quite similar to those imposed by GNU on Emacs documentation. Why is
this manual licensed differently (under the GNU General Public License,
or GPL), and why does it have to be distributed separately from
the XEmacs User's Guide and the XEmacs Lisp Reference Manual?

Taking the second question first, XEmacs is community-owned
software. That is, unlike GNU Emacs, there is no monopoly copyright
holder. Many of us, including the original Lucid authors, have
contributed our copyrights to the Free Software Foundation (FSF), and of
course much content is derived from GNU Emacs, and therefore is held by
the FSF. Another large chunk is held by Sun Microsystems, and a few
individual authors hold copyright to thousands of lines each. But many
individuals hold copyright to only a few dozen lines. Like the Linux
kernel, copyright ownership is distributed throughout a community.

However, its license is "copyleft," i.e., it requires
that you redistribute it under terms identical to those under
which you received it, unless you have explicit permission of the
copyright holder. Because of the multiple owners, determining the
ownership of any given part of XEmacs is tedious, and perhaps
impossible. For practical purposes, then, the license of any
substantial chunk of existing XEmacs content cannot be changed, except
to a later version of the GPL, for those parts under GPL. (That is due
to the explicit permission to change to a later version of the
GPL, present in every file of XEmacs.)

Unfortunately, this severe restriction means that the GPL, FDL, and the
XEmacs documentation license (XDL) are mutually
incompatible. That is, content licensed under any of the GPL, FDL, or
XDL may not be mixed with content licensed under either of the
other two without changing the license of some of the content. But this
requires permission of the copyright holder, which is often difficult or
impossible to get.

For example, you may not take comments or docstrings from XEmacs
code and add them to the Lispref to mend a gap in the latter's coverage.
You may not copy text from the Lispref into docstrings in the
code. And you may not copy text from the GNU Emacs Lisp
Reference to the XEmacs Lisp Reference Manual. (In this case it is at
least trivial to ask permission, although it is rather unclear whether
it would be granted.)

In fact, parts of this document were derived by copying from XEmacs code
under the GPL, without any further permission from the authors. Thus,
this document must be distributed under the GPL, as a "volume"
separate from the XEmacs documentation under the XDL. Note that the
"mere aggregation" clauses allow us to distribute in the same
tarball. But incorporating it as a node in the Lispref is prohibited,
even if done by inclusion.

A bit of advocacy:

If you look carefully at the additional restrictions imposed by the
soi-disant "free" documentation licenses, you discover that they are
simply proprietary restrictions guaranteeing a certain amount of
unpaid political advertising to the Free Software Foundation and
GNU Project (and in the case of the FDL, this is extended to commercial
advertising by authors of original or derived works). Whether this is
"ethically justified" or not is a difficult question. What is certain
is that there is little social benefit to these terms (since the license
documents themselves contain the advocacy and must be included with any
distribution).

I conclude it makes sense for XEmacs to reduce its restrictions, where
possible, to the "least common denominator," the GNU General Public
License.